I am not entirely clear about what is going on. So I am going to ask some questions for clarification.
Wife left the bath gray tank cracked just a little.
Did you leave a faucet dripping that would drain into the gray tank? Was the gray tank valve partially open, fully open?
Was solid ice the whole dump pipe, got what I can see thawed with a blow dryer and currently have a blow dryer into the pipe.
Are you referring to the PVC pipe under the trailer or the attachable sewer hose that lays on the ground? If it's the pvc pipe under the trailer how do you know it's ice the whole way up?
It's only 25deg out and when I go to dump gray tank nothing still comes out.
Can you easily move the dump valve? Is it possible the tank is empty?
With blow dryer into the outlet pipe water is still dripping out.
Be a little careful with that, if there is blockage, and there is water in the tank, and the valve is open, then it is possible to soak the dryer causing it to short out. Not a given this will happen, just something to be aware of.
How deep can this ice melt go with tank heaters on? Ay other suggestions what to do?
When you say ice melt are you referring to the warm air from the hair dryer, or the amount of melt (warm water) produced by the tank heaters, or some other form of ice melt?
Tank heaters are flat pads that are about 12” wide and 20” long. They are placed underneath the tank near the valve. They only warm to about 40f, so not real hot. If water is frozen inside the pvc pipe they will not assist in the melting.
You say there is dripping. Can you determine if that is from melting caused by the hair dryer? If so you are still plugged. If there is dripping come from the tank and an open valve, that will increase the rate of melt and eventually open the pipe, particularly if you can establish water flow through the system. But if you still completely plugged add more water does not help, just makes more ice.
If you have a space heater, an old blanket, and an extension cord. You can place the space heater under the exposed pvc pipe, wrap the blanket around it, and let it set for about an hour. Be very careful to leave space between the heater and the blanket, we don't want to catch the blanket on fire and burn the RV down. If in doubt, don't insulate it with a blanket, just let the space heater run near the pipes. Check for hot spots so nothing gets too warm.
Let us know how it's going....